Question

write about molicular orientation?

Answer 1

As polymers are processed and shaped by flowing into moulds the shear stress fields induce preferred orientations in the molecules. The hydrostatic components of the stress field cause packing. These orientation and packing effects will relax with time if the temperatures are high enough, but the moulding cycle is frequently such that they are ‘frozen-in’ by cooling or perhaps fixed into the structure because the material has been crosslinked. The consequent moulded-in or residual stresses and strains may

subsequently warp the moulding and

can increase the likelihood of fracture or cracking, particularly in the presence of some hostile chemicals.

Molecular orientation due to moulding sometimes results in physical properties which vary significantly with direction, i.e. the properties are anisotropic. This can be beneficial and is therefore sometimes induced deliberately. For example, when synthetic fibres are spun they are oriented uniaxially to increase the strength in the fibre direction. In this way, the high potential strength and stiffness of carbon-carbon bonds in the backbone chain of linear high polymers can be achieved if the chains can be fully aligned along the fibre axis. This has been achieved in aramid fibres and other aliphatic fibres also show significant improvements although nowhere near as great as with aramid fibres like 

Answer 2

Molecular orientation can be brought about by melt flow, for example during injection molding or extrusion, or by cold flow during stretching processes performed on monofilament or film.

Orientation

Mechanical Properties of Polymeric Materials

E. Alfredo Campo, in Selection of Polymeric Materials, 2008

2.6 Molecular Orientation Effects

Molecular orientation affects the tensile strength values. A load applied parallel to the direction of molecular orientation may yield higher values than the load applied perpendicular to the orientation. The opposite is true for elongation.

The process employed to prepare the specimens also has a significant effect. For example, an injection molded specimen generally yields higher strength values when it is subjected to flexural or compression loads rather than tensile or shear loads. Another important factor affecting the test results is the number, location, and size of the gate on the molded specimen. This is especially true for glass fiber reinforced tensile test specimens. A large gate located on top of the tensile specimen will orient the fiberglass parallel to the applied load, yielding higher tensile strength. A gate located on one side of the tensile test specimen will disperse the fiberglass in different directions.